Not too long ago, leeches were all the rage throughout Europe. Umm, ok, in America too. But to a far lesser extent. This is due to the misplaced but long-held notion that human health depended upon maintaining balance between the four humours -- black bile, yellow bile, phlegm and blood. Of all those humours, it was blood that received the most attention. If it was decided that you either had "bad blood" or too much blood, the medical people of the day would use a leech or two to remove it. This practice was known as "blood letting".

Leeches are specialised predatory or parasitic worms (subclass Hirudinea) that feed on other animals. A few species are terrestrial, but most are aquatic, with the vast majority of those living in fresh water.

True leeches (Euhirudinea) are charming creatures with suckers on each end of their bodies (although they typically only feed using their anterior sucker, the posterior sucker strengthens their attachment to their host). Similar to segmented worms (Annelida), to which they may or may not be closely related, leeches are hermaphrodites, so each individual is both a Larry and a Lucia. Unfortunately for leeches, they are not their own best friend, which means they still have to actively seek out a mate so they can reproduce.

Leeches are subdivided into two major groups based upon the structure of their feeding apparatus. Although most leech species are carnivorous, and their feeding apparatus are designed specifically for preying upon small invertebrates, a minority of leeches are parasites that feed by sucking blood from other animals. Most of these blood-sucking species are unable to bite; feeding either upon the blood of dead animals or already open wounds on a living animal.

The most famous of all blood-sucking leeches are the so-called medicinal leeches, Hirudo medicinalis. They CAN bite: Their anterior sucker is comprised of jaws made up of three parts and armed with hundreds of tiny, very sharp teeth. This jaw arrangement leaves a distinctive mark on the skin that looks like a "Y" within a circle.

Whilst feeding, leeches secrete an anticoagulant enzyme, hirudin, that keeps the blood from clotting. Hirundin is a small protein, comprised of just 65 amino acid residues, that binds to, and thereby inhibits, the activity of thrombin, which is part the blood-clotting cascade. Hirudin has inspired the development of a number of anticoagulant recombinant pharmaceuticals that are in use today. But medicinal leeches have also made a comeback in modern medicine, because they can be applied to specific areas of the body -- particularly to skin grafts and in some types of reconstructive surgeries -- where their bites stimulate healing by increasing the flow of nourishing blood.

Here's an interesting video, courtesy of the Royal Institution, that tells us a little more about medicinal leeches: